1. Field of Invention
The present invention relates to an optical module structure, and more particularly, to a parallel optical subassembly module structure for a parallel optical data link.
2. Related Art
The parallel optical data link as one of the new methods of transmission has been developed to respond to the shift of fiber communication from a long-distance backbone network to clients.
The so-called parallel optical data link is to transmit parallel optical signals Point-to-Point without doing any parallel/serial switch so as to transmit optical signals in parallel through different fibers at high speed to achieve high bandwidth required by multi-clients in access network.
The parallel optical data link can be practically applied to the board-to-board data link in the back plane of communication apparatus, as well as to the data link between multi-processors and peripheral of high-level servers in LAN (For example: data transmission in the memory from one chip to another) to break the bottleneck of data transmission.
The key devices of parallel optical subassembly module applying to parallel optical data link are the VCSEL array in the optical signals emitting part and the PD array in the optical signals receiving part. The structure of the module depends on either optical turn or electrical turn in operation.
FIG. 1A illustrates a structure of the optical turn in optical signals emitting part, and FIG. 1B illustrates a structure of the electrical turn in optical signals emitting part. Referring to FIG. 1A, An optical signal emitted from a VCSEL array 10 couples with a fiber array 30 after reflecting 90° by a reflecting mirror 20. The reflecting mirror 20 is used to perform the optical turn.
Referring to FIG. 1B, for the characteristics of vertical light-emitting of VCSEL array 10, most of the parallel optical subassembly modules using VCSEL array 10 as a light source take the structure of electrical turn to make the lighting area of the VCSEL array 10 face the fiber array 30 directly to couple light.
In the structure of an optical turn, many devices are required to achieve an optical turn, and thereby the tolerance of fabricating devices and the reliability of production is reduced. Besides, the structure of an optical turn is quite unfeasible and has difficulties in mass production.
Furthermore, since some optical devices are made from optical plastic materials with a relatively high coefficient of thermal expansion (CTE) to undergo processes of injection molding, the devices dilate and deform under thermal loading, and the coupling efficiency of devices deteriorate as a result.
As for the electrical turn, it is preferable to accomplish the electrical turn by flexible printed circuit board (FPC) as disclosed in the U.S. Pat. No. 6,203,212, instead of a lead frame or electroplating trace pattern with 90° turn, since FPC with better flexibility of fabrication matches the impedance.
In the U.S. Pat. Application Publication No. 2003/0113071 A1 disclosed a subassembly for use in fiber optic communications. The subassembly also uses a flexible circuit to connect a carrier assembly and a circuit board in order to accomplish the electrical turn. Because the signal quality at high frequency transmission of the ceramic substrate is better than that of the FPC, efficiency of the assembly is better than that with all FPC. Some optical devices and the driver IC are all set on the carrier assembly. This design will increase the height and the volume of the transceiver module. Besides, the heat generation power of the driver IC is extremely high. This will increase the temperature of some area. The optical devices made of high CTE material and the carrier assembly will deform because of the CTE mismatch between the optical devices and the carrier assembly as mentioned before. This will affect the coupling efficiency.
As for the electrical turn, when the FPC is bended with a large angle (90°), it is hard to evade the impedance discontinuity. Especially in the case of high frequency transmission, there is reflection noise at the bending part of the FPC. The method of increasing the curvature radius of the bending part of the FPC to diminish negative effects caused by the bending would lengthen the trace of the circuit, and thereby worsen the transmission of high frequency signals. So a transmission structure of the optical turn is necessary.